Sound strategy for levitating and moving particles

Bounce a propagating sound wave off an acoustic mirror and the superposition of original and reflected waves may yield a standing pressure wave. A particle located in that pressure field will feel a force driving it to a position of stable equilibrium; indeed, an acoustic standing wave with sufficient amplitude can even suspend a particle against the action of gravity. Numerous researchers have used acoustic levitators to study behaviors of liquid drops without the complications of a confining vessel. The problem with conventional levitators, though, is that the separation between the wave generator and the reflector must be resonantly tuned if a standing wave is to result. Now Marco Andrade of the Institute of Physics at the University of São Paulo, Brazil, and colleagues have demonstrated a device that can levitate particles— indeed, manipulate them—without the need for fine tuning. Their trap, based on a decades-old proposal by Charles Rey, is shown here suspending four polystyrene balls. It comprises a 10-mm-diameter cylindrical transducer (at the tops of the photos) that generates the sound, and a somewhat larger concave reflector. For the coaxial geometry shown on the left, the trapped balls remain at a fixed distance from the mirror even as the mirror–transducer separation varies from 50 mm to 100 mm. Moreover, as one misaligns the axes of the transducer and mirror (right photo), the balls will adjust their positions, remaining confined all the while. (M. A. B. Andrade, N. Pérez, J. C. Adamowski, App. Phys. Lett. 106, 014101, 2015, doi:10.1063/1.4905130 .)